One problem with sentences which contain idioms is that they are typically ambiguous,
in the sense that either a literal or idiomatic interpretation is generally possible (i.e. the
phrase kick the bucket can really be about buckets and kicking). However, the possibility
of having a variety of interpretations does not really distinguish them from other sorts of
expression. Another problem is that they need special rules (such as those above, perhaps),
in addition to the normal rules for ordinary words and constructions. However, in this they
are no different from ordinary words, for which one also needs special rules. The real
problem with idioms is that they are not generally fixed in their form, and that the variation
of forms is not limited to variations in inflection (as it is with ordinary words). Thus, there
is a serious problem in recognising idioms.